High efficiency back contact type solar cell, solar cell module, and photovoltaic power generation system

ABSTRACT

In a back contact type solar cell in which an impurity diffusion layer where second conductive type impurities are diffused is formed on a back surface, as a non-light receiving surface, of a first conductive type semiconductor substrate, and an electrode in contact with the impurity diffusion layer is provided, a surface concentration of the impurities in the impurity diffusion layer is not less than 5×1017 atms/cm3 and not more than 5×1019 atms/cm3, and a diffusion depth of the impurities in the impurity diffusion layer is not smaller than 1 μm and not larger than 2.9 μm from a top of the back surface. It is thereby possible to provide a high efficiency back contact type solar cell which can be manufactured by a simple method at low cost.

TECHNICAL FIELD

The present invention relates to a high efficiency back contact typesolar cell having favorable conversion efficiency, a solar cell module,and a photovoltaic power generation system.

BACKGROUND ART

A solar cell is typically made of multi-crystalline silicon,single-crystalline silicon, or the like, in a plate shape with a size of100 to 150 mm square and a thickness of 0.1 to 0.3 mm, and a mainmaterial for the solar cell is a p-type semiconductor substrate dopedwith p-type impurities such as boron. In this solar cell, an n-typediffusion layer (emitter layer) and an antireflection film are formed ona light receiving surface that receives sunlight, and an electrode isformed penetrating through the antireflection film so as to be incontact with the emitter layer.

In the solar cell, the electrode is essential for taking out a currentobtained by photovoltaic conversion. However, since sunlight cannotenter the cell beneath the electrode regions on the light receivingsurface due to shielding by the electrode, the larger the area of theelectrode, the more the conversion efficiency degrades and the currentdecreases. Such a loss of the current due to the electrode formed on thelight receiving surface is called a shadow loss.

In contrast, aback contact type solar cell has no shadow loss, with noelectrode formed on the light receiving surface, and can thus absorbtherein almost 100% of incident sunlight except for a small amount ofreflected light that the antireflection film failed to prevent fromreflection. It is thus possible in principle to achieve high conversionefficiency.

Typically, a back contact type solar cell 100 has a sectional structureas illustrated in FIG. 1. The back contact type solar cell 100 includesa semiconductor substrate 101, an emitter layer 104, a BSF (Back SurfaceField) layer 106, antireflection films with passivation properties 107,108, and electrodes 109, 110.

The semiconductor substrate 101 is a main material for the back contacttype solar cell 100, and made of single-crystalline silicon,multi-crystalline silicon, or the like. While either a p-type substrateor an n-type substrate may be used, an n-type silicon substrate dopedwith n-type impurities such as phosphorus is often used. Hereinafter, adescription will be given taking as an example the case of using then-type silicon substrate. For the semiconductor substrate 101, asubstrate in a plate shape with a size of 100 to 150 mm square and athickness of 0.1 to 0.3 mm is preferred, and one main surface is used asa light receiving surface, and the other main surface is used as anon-light receiving surface (back surface).

A concave-convex structure for optical confinement is formed on thelight receiving surface. The concave-convex structure is obtained bysoaking the semiconductor substrate 101 in an acid or alkali solutionfor a certain period of time. Typically, this concave-convex structureis called a texture.

The back surface is formed with the emitter layer 104 being a p-typediffusion layer doped with p-type impurities such as boron, and the BSFlayer 106 being an n-type diffusion layer doped with n-type impuritiessuch as phosphorus. Either the emitter layer 104 or the BSF layer 106may be formed first. When the emitter layer 104 is to be formed first,for example, it is formed in the following manner.

First, a protective layer 102 such as a silicon oxide layer is formed onthe entire surface of the semiconductor substrate 101. Specifically, asilicon oxide layer with a thickness of approximately 30 to 300 nm isformed by, for example, a thermal oxidation method in which thesemiconductor substrate 101 is set in an oxygen atmosphere at a hightemperature of 800 to 1100° C. Subsequently, by screen printing, resistpaste is applied to the regions of the protective layer 102 other thanregions for forming the emitter layer 104 on the back surface of thesemiconductor substrate 101, and the resist paste is then cured. Thesemiconductor substrate 101 is then soaked into a hydrofluoric acidaqueous solution to remove the protective layer 102 covering the regionfor forming the emitter layer 104, and is further soaked into acetone orthe like to remove the resist paste 103. Then, p-type impurities arediffused by, for example, the thermal diffusion method in the regionwhere the protective layer 102 has been removed, to form the emitterlayer 104 as a p-type diffusion layer and a glass layer 105.Specifically, for example by placing this semiconductor substrate 101 ina high temperature gas at 800 to 1100° C. containing BBr₃, boron isdiffused in the regions not formed with the protective layer 102, toform the glass layer 105 and the emitter layer 104 with sheet resistanceof approximately 20 to 300Ω/□. The semiconductor substrate 101 is thensoaked into a chemical such as a diluted hydrofluoric acid solution toremove the remaining protective layer 102 and the glass layer 105, andcleaned by deionized water. This leads to formation of the emitter layer104 where p-type impurities are diffused in a desired region on the backsurface of the semiconductor substrate 101.

Then, the BSF layer 106 is formed in the portion not formed with theemitter layer 104 on the back surface of the semiconductor substrate 101in almost a similar procedure to the emitter layer 104.

The antireflection films with passivation properties 107, 108, made ofSiN (silicon nitride) or the like are further formed on the lightreceiving surface formed with the texture and the back surface formedwith the emitter layer 104 and the BSF layer 106, respectively.

The electrode 109 is formed so as to be in contact with the emitterlayer 104, and the electrode 110 is formed so as to be in contact withthe BSF layer 106. These electrodes may be formed by sputtering or thelike after opening contacts by using etching paste or the like, or maybe formed by using the screen printing method. In the case of using thescreen printing method, conductive silver paste containing glass flit orthe like is printed on the antireflection film with passivation property108 and dried so as to be in contact with the emitter layer 104 and theBSF layer 106 after firing. By firing the conductive silver paste, theelectrode 109 in contact with the emitter layer 104 and the electrode110 in contact with the BSF layer 106 are respectively formedpenetrating through the antireflection films with passivation properties107, 108. The electrodes 109, 110 are each made up of a bus barelectrode for externally taking out a photo-generation current generatedin the back contact type solar cell 100, and a current-collecting fingerelectrode in contact with the bus bar electrode (illustration omitted).

In the typical back contact type solar cell having the structureillustrated in FIG. 1, especially an impurity diffusion profile of theemitter layer has a great influence on the conversion efficiency of thesolar cell. For example, reducing an amount of the impurities diffusedinto the emitter layer to lower a reverse saturation current densitymakes it possible to increase an open voltage of the solar cell andenhance the conversion efficiency thereof. However, when a surfaceconcentration of the impurities decreases due to reduction in amount ofthe impurities diffused into the emitter layer, the contact resistancewith the electrode in contact with the emitter layer typically increasesto cause deterioration in conversion efficiency. For this reason, it hasbeen considered preferable for improving the conversion efficiency touse a method of setting the diffusion profile where the surfaceconcentration of the impurities is high and diffusion depth is smallwhile the amount of the impurities diffused into the emitter layer isheld small, to make the contact resistance as small as possible and alsokeep the reverse saturation current density from increasing within thepossible range. In the case of using the above exemplified method formanufacturing the emitter layer, the emitter layer is often formed inthe diffusion profile where the surface concentration of the impuritiesis high and the diffusion depth thereof is small.

The contact resistance between the emitter layer and the electrode isgreatly influenced also by the size of the contact area between theemitter layer and the electrode. In the case of the conventional solarcell having the electrode on the light receiving surface, the fingerelectrode in contact with the emitter layer needs being thinned tominimize the electrode area so as to make the shadow loss caused by theelectrode as small as possible. This makes sufficient reduction incontact resistance difficult and makes formation of the electrodecostly. In contrast, in the back contact type solar cell, the electrodeis formed on the non-light receiving surface, thereby eliminating theneed for considering the shadow loss caused by the electrode. Hence inthe case of the back contact type solar cell, it has been consideredpreferable to make the width of the finger electrode large to a certainextent so as to make the contact area between the emitter layer wide andthe contact resistance small, and preferable to form a thin electrode soas to make a cross-sectional area (an area of a plane orthogonal to thecontact surface) small to such an extent that lateral flow resistancedoes not become excessively large in order to keep the cost for formingthe electrode low. In the case of the previously illustrated method formanufacturing the electrode by screen printing, an electrode having alarge width and a small thickness is often formed.

However, in order to diffuse the impurities in the diffusion profilewhere the surface concentration of the impurities is high and thediffusion depth thereof is small, it is generally necessary to performdiffusion thermal treatment at high temperature for a short period oftime. In this case, especially at the time of mass-production,non-uniformity in diffusion of the impurities is apt to occur, thus itis difficult to diffuse the impurities in a predetermined diffusionprofile. Further, the electrode with a large width and a small thicknessmay accidentally increase a value of the contact resistance with theemitter layer, which has frequently caused a decrease in yield.

As countermeasures against such a problem, for example, Patent Document1 discloses a method for manufacturing a solar cell in which a filmcontaining a diffusion source is formed in a portion of a diffusionlayer formed in a semiconductor substrate where an electrode is to beformed, and subjected to thermal treatment in a steam atmosphere, toform a highly concentrated diffusion layer only in a place immediatelybelow the electrode. However, this method has a problem where, since thethermal treatment needs to be performed a plurality of times for formingthe diffusion layer, the cost becomes high, and further, a lifetimekiller such as heavy metal is diffused in the semiconductor substrate toeasily cause a decrease in yield.

Patent Document 2 discloses a method for manufacturing a solar cell inwhich a dopant solution is applied to the substrate surface by ink jetprinting to perform uniform diffusion. However, in the case of thismethod, there has been a problem where nozzle and the dopant solutionare difficult to control in ink jet printing, and further, the diffusionsource formation and the thermal treatment are performed as differentsteps to cause an increase in cost.

Patent Document 3 discloses a method for manufacturing a solar cell inwhich conductive paste is screen-printed a plurality of times, while amask is replaced, to reduce resistance of an electrode. However, thismethod has a problem where a yield is apt to decrease due todisplacement of the electrode and the cost is apt to increase due to anincrease in amount of the conductive paste used.

Patent Document 4 discloses a method for manufacturing a solar cell inwhich an electrode is formed by so-called rotogravure printing to forman electrode accurately. However, this method has a problem where a rolland paste are difficult to control, and jam of the roll and a decreasein yield are apt to occur due to drying.

PRIOR ART REFERENCES Patent Documents Patent Document 1: WO 2015/151288Patent Document 2: JP 2003-168807 A Patent Document 3: WO 2011/111192Patent Document 4: JP 2011-049514 A SUMMARY OF THE INVENTION Problem tobe Solved by the Invention

An object of the present invention is to provide a high efficiency backcontact type solar cell with favorable conversion efficiency, a solarcell module, and a photovoltaic power generation system, the cell havinga small reverse saturation current density and small contact resistancebetween an emitter layer and an electrode, and being manufacturable by asimple method with good yield at low cost.

Means for Solving the Problems

(1) A high efficiency back contact type solar cell of the presentinvention is a high efficiency back contact type solar cell in which animpurity diffusion layer where second conductive type impurities arediffused is formed on a back surface, as anon-light receiving surface,of a first conductive type semiconductor substrate, and an electrode incontact with the impurity diffusion layer is provided. In the solarcell, a surface concentration of the impurities in the impuritydiffusion layer is not smaller than 5×10¹⁷ atms/cm³ and not larger than5×10¹⁹ atms/cm³, and a diffusion depth of the impurities in the impuritydiffusion layer is not smaller than 1 μm and not larger than 2.9 μm froma top of the back surface.

As thus described, by forming the emitter layer with a low surfaceconcentration and a large diffusion depth as compared with theconventional one, it is possible to lower both the reverse saturationcurrent density and the contact resistance, and thereby to achieve ahigh efficiency back contact type solar cell by a simple method at lowcost.

(2) Sheet resistance of the impurity diffusion layer may be not smallerthan 60Ω/□ and not larger than 150 Ω/□.

(3) A maximum value of an impurity concentration of the impuritydiffusion layer may be not lower than 7×10¹⁷ atms/cm³ and not higherthan 7×10¹⁹ atms/cm³. The impurity concentration of the impuritydiffusion layer may become the maximum value at a position not less than0.1 μm and not more than 1 μm deep from the top of the back surface. Theelectrode may be a sintered body containing at least glass flit, silver,and aluminum. A cross-sectional area of the electrode may be not smallerthan 350 μm² and not larger than 1000 μm². The electrode may partiallypenetrate into the impurity diffusion layer, and a depth of thepenetration may be not smaller than 0.1 μm and not larger than 1.9 μmfrom the top of the back surface.

It is thereby possible to suppress the contact resistance low to enhancethe conversion efficiency, and also to suppress the manufacturing costlow.

(4) For example, the first conductive type may be an n-type, and thesecond conductive type may be a p-type.

(5) A solar cell module may be configured by electrically connecting aplurality of high efficiency back contact type solar cells.

(6) A photovoltaic power generation system may be configured byelectrically connecting a plurality of solar cell modules.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view that illustrates one example of a configuration of ahigh efficiency back contact type solar cell.

FIG. 2 is a flowchart that illustrates a method for manufacturing thehigh efficiency back contact type solar cell.

FIG. 3 is a graph that illustrates one example of a relation between adepth from a substrate surface and an impurity concentration at thatdepth.

FIGS. 4(a) and 4(b) are graphs that plot reverse saturation currentdensities of the respective samples versus a surface concentration andsheet resistance, respectively.

FIGS. 5(a) and 5(b) are graphs that plot contact resistances of therespective samples versus the surface concentration and the sheetresistance, respectively.

FIG. 6 is a graph that illustrates a relation between a cross-sectionalarea of the electrode and the contact resistance in each sample.

FIG. 7 is a schematic view that illustrates a configuration example of asolar cell module configured using the high efficiency back contact typesolar cells of the present invention.

FIG. 8 is a schematic view that illustrates a configuration example ofthe back surface of the solar cell module illustrated in FIG. 7.

FIG. 9 is a schematic view that illustrates a configuration example of across section of the solar cell module illustrated in FIG. 7.

FIG. 10 is a schematic view that illustrates a configuration example ofa photovoltaic power generation system configured using the solar cellmodule illustrated in FIG. 7.

DESCRIPTION OF EMBODIMENTS

Hereinafter, embodiments of the present invention will be described. Acommon constituent among the drawings including the drawings used fordescribing the prior art is provided with the same reference numeral.

A high efficiency back contact type solar cell 200 in the presentinvention has a similar structure to that of the conventional backcontact type solar cell 100 illustrated in FIG. 1, and includes asemiconductor substrate 101, an emitter layer 104, a BSF layer 106,antireflection films with passivation properties 107, 108, andelectrodes 109, 110. The semiconductor substrate 101 is a main materialfor the high efficiency back contact type solar cell 200, and made ofsingle-crystalline silicon, multi-crystalline silicon, or the like.While either a p-type or an n-type may be used, in this text, adescription will be given taking as an example the case of an n-typesilicon substrate containing impurities such as phosphorus and having aspecific resistance of 0.1 to 4.0 Ω·cm. For the semiconductor substrate101, a substrate in a plate shape with a size of 100 to 150 mm squareand a thickness of 0.05 to 0.30 mm is preferred, and one main surface isused as a light receiving surface, and the other main surface is used asa non-light receiving surface (back surface).

<Manufacturing Procedure>

FIG. 2 illustrates a method for manufacturing the high efficiency backcontact type solar cell 200 of the present invention. Prior to themanufacturing, the semiconductor substrate 101 is soaked into an acidsolution or the like for damage etching, to remove damage on the surfacecaused by slicing or the like, and the semiconductor substrate 101 isthen cleaned and dried.

First of all, the emitter layer 104 is formed on the back surface of thesemiconductor substrate 101 after the damage etching (S1). Theprotective layer 102 such as a silicon oxide layer is first formed onthe entire surface of the semiconductor substrate 101 (S1-1). A siliconoxide layer with a layer thickness of approximately 30 to 300 nm isformed by, for example, a thermal oxidation method in which thesemiconductor substrate 101 is set in an oxygen atmosphere at a hightemperature of 800 to 1100° C. for a short period of time. Subsequently,by screen printing, resist paste is applied to the region of theprotective layer 102 other than regions for forming the emitter layer104 on the back surface of the semiconductor substrate 101, and theresist paste is cured (S1-2). The semiconductor substrate 101 is thensoaked into a hydrofluoric acid aqueous solution to remove theprotective layer 102 covering the region for forming the emitter layer104 (S1-3), and is further soaked into acetone or the like to remove theresist paste 103 (S1-4). Then, p-type impurities are diffused by, forexample, the thermal diffusion method in the region where the protectivelayer 102 has been removed, to form the emitter layer 104 as a p-typediffusion layer and a glass layer 105 (S1-5). Specifically, for exampleby placing this semiconductor substrate 101 in a high temperature gas at800 to 1100° C. containing BBr₃, boron is diffused in the regions notformed with the protective layer 102, to form the glass layer 105 andthe emitter layer 104 with sheet resistance of approximately 20 to300Ω/□. The semiconductor substrate 101 is then soaked into a chemicalsuch as a diluted hydrofluoric acid solution to remove the remainingprotective layer 102 and the glass layer 105, and is cleaned bydeionized water (S1-6). This leads to formation of the emitter layer 104where p-type impurities are diffused in a desired region on the backsurface of the semiconductor substrate 101.

Subsequently, the BSF layer 106 is formed in the region not formed withthe emitter layer 104 on the back surface of the semiconductor substrate101 (S2). First, the protective layer 102 such as a silicon oxide layeris formed on the entire surface of the semiconductor substrate 101(S2-1). A silicon oxide layer is formed by, for example, a thermaloxidation method in which the semiconductor substrate 101 is set in anoxygen atmosphere at a high temperature of 800 to 1100° C. At this time,the time for placement at the high temperature is made longer, to formthe silicon oxide layer, and simultaneously allow boron, diffused in thevicinity of the surface of the semiconductor substrate 101 at the timeof forming the emitter layer 104, to be diffused more deeply in thesemiconductor substrate 101 and also diffused in the formed siliconoxide layer. As a result, a surface concentration of boron forming theemitter layer 104 decreases, and the sheet resistance changesaccordingly. The diffusion depth, the surface concentration, and thesheet resistance can be appropriately adjusted by changing thermaloxidation conditions.

Subsequently, by screen printing, resist paste is applied to a region ofthe protective layer 102 which covers the region formed with the emitterlayer 104 on the back surface of the semiconductor substrate 101, andthe resist paste is cured (S2-2). The semiconductor substrate 101 isthen soaked into a hydrofluoric acid aqueous solution to remove theprotective layer 102 covering the region not formed with the emitterlayer 104 (S2-3), and is further soaked into acetone or the like toremove the resist paste 103 (S2-4). Then, n-type impurity elements arediffused by, for example, the thermal diffusion method in the regionwhere the protective layer 102 has been removed, to form the BSF layer106 as the n-type diffusion layer and the glass layer 105 (S2-5).Specifically, for example by setting this semiconductor substrate 101 ina high temperature gas at 850 to 1100° C. containing POCl₃, phosphorusis diffused in the region not formed with the protective layer 102, toform the glass layer 105 and the BSF layer 106 with sheet resistance ofapproximately 30 to 300Ω/□. The semiconductor substrate 101 is thensoaked into a chemical such as a diluted hydrofluoric acid solution toremove the remaining protective layer 102 and the glass layer 105, andis cleaned by deionized water (S2-6). This leads to formation of the BSFlayer 106 where n-type impurities are diffused in the region not formedwith the emitter layer 104 on the back surface of the semiconductorsubstrate 101.

Next, a concave-convex structure, which is called a texture, is formedon the light receiving surface of the semiconductor substrate 101 (S3).The texture can be formed by soaking the semiconductor substrate 101 inan acid or alkaline solution for a certain period of time. For example,the texture can be formed by applying resist paste to the entire backsurface of the semiconductor substrate 101 by screen printing, curingthe resist paste, and then chemically etching the back surface by use ofa potassium hydroxide aqueous solution or the like, followed by cleaningand drying. By forming the texture, incident light from the lightreceiving surface multiply reflects to be confined in the semiconductorsubstrate 101, thereby enabling effectively reduction in reflectance andimprovement in conversion efficiency. Thereafter, the semiconductorsubstrate 101 is soaked into acetone or the like to remove the resistpaste applied to the entire back surface of the semiconductor substrate101. Note that the texture may be formed before formation of the emitterlayer 104 and the BSF layer 106. The texture may also be formed on theback surface of the semiconductor substrate 101. In addition, an FSF(Front Surface Field) layer may further be formed on the light receivingsurface of the semiconductor substrate 101.

Subsequently, the antireflection films with passivation properties 107,108 made of SiN (silicon nitride) or the like are respectively formed onboth surfaces of the semiconductor substrate 101 (S4). In the case ofthe silicon nitride layer, it is formed by, for example, a plasma CVDmethod where a mixed gas of SiH₄ and NH₃ is diluted by N₂ andplasma-gasified by glow discharge decomposition, or by some othermethod. Each of the antireflection films with passivation properties107, 108 is formed such that a refractive index is approximately 1.8 to2.3 and the thickness is approximately 50 to 100 nm in view of adifference in refractive index from the semiconductor substrate 101.This film performs the function of preventing the reflection of light onthe front surface of the semiconductor substrate 101 and effectivelyabsorbing the light in the semiconductor substrate 101, and alsofunctions as a passivation layer having a passivation effect on then-type diffusion layer, to exert the effect of improving electriccharacteristics of the solar cell. Note that the antireflection filmswith passivation properties 107, 108 may be a single-layered film ofsilicon oxide, silicon carbide, amorphous silicon, aluminum oxide,titanium oxide, or the like, or a laminated film formed by combiningthese. Different films may be used for the light receiving surface andthe back surface of the semiconductor substrate 101.

Subsequently, the electrodes 109, 110 are formed (S5). The electrode maybe formed by, for example, providing openings in the antireflectionfilms with passivation properties 108 by etching paste or the like andperforming sputtering, or may be formed by the screen printing method.In the case of using the screen printing method, first, conductive pastecontaining, for example, a silver powder, glass frit, an aluminumpowder, or vanish is screen-printed on each of a regions of theantireflection film with passivation property 108 where the electrode109 in contact with the emitter layer 104 is to be formed and regionsthereof where the electrode 110 in contact with the BSF layer 106 are tobe formed, and the conductive paste is then dried. At this time, a meshroughness, an emulsion thickness, an opening size, and the like of ascreening mask, which is used in screen printing, are changed to allowappropriate adjustment of the width and the cross-sectional area of theelectrode.

The conductive paste printed as above is fired at approximately 500° C.to 950° C. for approximately 1 to 60 seconds to penetrate through theantireflection film with passivation property 108 (firing through).Thereby, a sintered body containing silver, glass flit, and aluminum iselectrically connected with the emitter layer 104 or the BSF layer 106to be formed into the electrodes 109, 110. Note that the firing at thetime of forming the electrode may be performed once or may be separatelyperformed more than once. Further, the conductive paste to be appliedonto the emitter layer 104 and the conductive paste to be applied ontothe BSF layer 106 may be different.

<Consideration of Problem Solving Means>

By the above manufacturing method, a plurality of samples for measuringa reverse saturation current density of the emitter layer were producedwhile changing the thermal oxidation conditions. Note that the electrodewas not formed since the electrode is unnecessary in measuring thereverse saturation current density. A specific resistance of the n-typesemiconductor substrate used for the production is 1 ω·cm, a dopingconcentration of phosphorus is about 5×10¹⁵ atoms/cm³, and theantireflection films with passivation properties are a silicon nitridelayer having a thickness of 100 nm and formed by the plasma CVD methodusing SiH₄ and NH₃, and N₂.

In each sample, a diffusion profile of boron into the emitter layer wasmeasured by SIMS (equipment used was “ims-4f” manufactured by CAMECASAS., primary ion was O₂ ⁺, primary ion energy was 10.5 keV, a scanningregion was 200×200 μm, a detected region was 104 μmφ, secondary ionpolarity was positive). FIG. 3 is a graph that illustrates a relationbetween a depth from the substrate surface and a concentration of boronbeing impurities at that depth, obtained as a measurement result of acertain sample. As viewed from FIG. 3, a sudden extreme fluctuation ofthe concentration is recognized around a depth of 0 to 0.02 μm in thevicinity of the outermost surface. This concentration fluctuation isconsidered as variation in measured value influenced by a foreign matteron the substrate surface, the unevenness of the substrate surface, orthe like, and includes a number of errors. Accordingly, the surfaceconcentration is defined as a concentration around 0.03 μm in thefollowing. In case that such definition was made, the surfaceconcentrations of the respective produced samples were 1×10¹⁷ to 1×10²⁰atoms/cm³. Further, the diffusion depth is defined hereinafter as adepth at which a concentration of diffused boron obtained by the SIMSmeasurement becomes equal to a concentration of phosphorus doped intothe n-type semiconductor substrate. In case that such definition wasmade, the diffusion depths of the respective produced samples were 0.7to 3.5 μm.

After measurement of the diffusion profile, reverse saturation currentdensities of the respective samples for measuring a reverse saturationcurrent density were measured by the QSSPC method (equipment used wasWCT-100, manufactured by Sinton Consulting Inc.). FIGS. 4 (a) and 4 (b)are graphs that plot the reverse saturation current densities of therespective samples, obtained by QSSPC, versus the surface concentrationand the sheet resistance, respectively. FIG. 4 (a) is a graph that plotsthe current densities versus the surface concentration, and FIG. 4 (b)is a graph that plots the current densities versus the sheet resistance.From FIGS. 4 (a) and 4 (b), the reverse saturation current density isrecognized as having a tendency to become lower with decrease in surfaceconcentration, whereas it is found to have no correlation with the sheetresistance. That is, it can be said that lowering the surfaceconcentration allows an increase in open voltage, and thereby allowsenhancement of the conversion efficiency.

Next, conductive paste was applied to each of the samples for measuringa reverse saturation current density in a pattern based on the TLMmethod, and fired at 800° C. for ten seconds to produce a plurality ofsamples for measuring contact resistance. The TLM method is widely knownas a method for accurately measuring contact resistance of a contact ina mode where a current incident vertically on the contact surface turnsand flows horizontally on the device surface. FIGS. 5 (a) and 5 (b) aregraphs that plot contact resistances of the respective samples, measuredby the TLM method, versus the sheet resistance and the surfaceconcentration, respectively. From FIGS. 5(a) and 5 (b), the contactresistance is recognized as having a positive correlation with the sheetresistance, whereas it is found to have no correlation with the surfaceconcentration.

From the above, in order to achieve an emitter layer with both reversesaturation current density and contact resistance being low, it is onlynecessary to form an emitter layer with a low surface concentration andlow sheet resistance. This condition can be easily satisfied by formingan emitter layer with a low surface concentration and a large diffusiondepth. This is because, although the sheet resistance typicallyincreases when the surface concentration decreases, that increase can bereduced by increasing the diffusion depth.

Next, among the samples for measuring contact resistance, samples withsheet resistance of 70Ω/□ were formed by setting a width of a fingerelectrode connected with the emitter layer to a fixed value (60 μm) andchanging a cross-sectional area of the finger electrode, and the contactresistance was measured. In this case, the cross-sectional area of theelectrode was measured using a laser microscope VK-8500, manufactured byKEYENCE CORPORATION, and the contact resistance was measured using theTLM method. The cross-sectional area was changed by changing a thicknessof a gauze for a screen printing plate and a thickness of an emulsion.The contact resistance of the electrode as thus measured was plottedversus the cross-sectional area (FIG. 6). It is found from FIG. 6 that,even though the contact area of the electrode is fixed, when thecross-sectional area decreases, the contact resistance increases, andthe conversion efficiency deteriorates. It can thus be said that thecross-sectional area of the electrode is preferably made large in arange permitted by cost.

As one of reasons why the contact resistance becomes smaller when thecross-sectional area of the electrode is larger, the followingcircumstance can be cited.

When the conductive paste was fired to form the electrode, a place wasrecognized where the electrode penetrated into the substrate to a depthof approximately 2 μm at the maximum. In this context, the penetrationdepth was measured by soaking the electrode into hydrofluoric acid andnitric acid to remove the electrode from the substrate, and observing amark of the removal left on the substrate by SEM. In confirming therelation between the penetration depth and the contact resistance of theelectrode as to a plurality of samples, it was found that in theelectrode having large contact resistance with the emitter layer, thereis almost no penetrating and connected place, or even if there is such aplace in rare cases, the penetration is shallow, but in the electrodehaving small contact resistance with the emitter layer, there is a largepenetrating and connected area, and the penetration is deep. That is, itis considered that the deeper the electrode penetrates, the smaller thecontact resistance, and it is considered therefrom that the contactresistance depends not only on the dopant concentration of the surface,but also on a depthwise change in dopant concentration. Specifically, itis considered that the contact resistance can be reduced more by theelectrode penetrating to such a depth as to roughly cover a range inwhich the impurities are diffused with a high concentration.

However, when the cross-sectional area of the electrode is small at thistime, absolute amounts of materials such as glass frit, aluminum, andthe like which constitute the electrode and assist electric connectionwith the emitter layer, become insufficient, leading to an increase incontact resistance. It can thus be said that in order to avoid theincrease in contact resistance and obtain a stable yield, thecross-sectional area of the electrode is preferably made large in arange permitted by cost, while the electrode is made to penetrate intothe emitter layer to a certain extent.

EXAMPLES

The correctness of the problem solving means specified by the aboveconsideration was verified using a plurality of products manufactured bymaking the impurity diffusion profile and the electrode shape different.Manufacturing methods and verification results of the respective samplesare shown below.

<First Common Step>

An n-type silicon substrate, doped with phosphorus and produced byslicing to have a thickness of 0.2 mm, and made of n-typesingle-crystalline silicon with a resistivity of about 1 Ω·cm and adoping concentration of about 5×10¹⁵ atms/cm³, was prepared, andsubjected to outside diameter processing, to be formed into a squareplate shape with a side length of 15 cm. This substrate was soaked in afluonitric acid solution for 15 seconds to be subjected to damageetching, and thereafter cleaned with deionized water and dried.

The n-type silicon substrate after the damage etching was set in anoxygen atmosphere and thermally oxidized at a temperature of 1000° C.for 45 minutes, to form a silicon oxide layer on each surface of thesubstrate with a thickness of 50 nm. Then, the resist paste wasscreen-printed on a region at which the BSF layer was to be formed ofthe silicon oxide layer formed on the back surface of the substrate, andheated at a temperature of 100° C. to be dried. In this context, withthe emitter layer having a width of 800 μm and the BSF layer having awidth of 200 μm, a screen printing plate was formed in such a pattern asto have a structure of an interdigitated back contact cell where theemitter layers and the BSF layers were alternately formed. As the resistpaste, Paste 185 manufactured by Lektrachem Ltd. was used. The substratewas soaked into a 2% hydrofluoric acid aqueous solution to partiallyremove the silicon oxide layer while leaving the layer on the region atwhich the BSF layer was to be formed, and then soaked into acetone toremove the resist paste, and thereafter, the substrate was cleaned withdeionized water and dried. Next, thermal diffusion treatment wasperformed on the back surface of the substrate in a BBr₃ gas atmosphereat a temperature of 900° C. for 20 minutes, to form a p-type diffusionlayer as the emitter layer and a glass layer on the back surface of thesubstrate. The formed p-type diffusion layer had a sheet resistance ofabout 70Ω/□ and a diffusion depth of 0.5 μm. Thereafter, this substratewas soaked into a 25% hydrofluoric acid aqueous solution, and thencleaned with deionized water and dried to remove the silicon oxide layerand the glass layer.

Comparative Example 1

Comparative Example 1 is the case of adopting a method for manufacturinga conventional product where it takes a short period of time (45minutes) to perform thermal oxidation in the step of forming theprotective layer (silicon oxide layer) which is performed afterformation of the emitter layer. Specifically, after performing thefollowing step, second and third common steps described later areperformed to manufacture a back contact type solar cell.

The substrate formed with the emitter layer as described above was setin an oxygen atmosphere and thermally oxidized at a temperature of 1000°C. for 45 minutes, to form a silicon oxide layer on each surface of thesubstrate with a thickness of 50 nm. By the thermal treatment at thistime, boron diffused in the emitter layer was re-diffused. A diffusionprofile of boron in the emitter layer after the re-diffusion wasmeasured by SIMS (equipment used was “ims-4f” manufactured by CAMECASAS., primary ion was O₂ ⁺, primary ion energy was 10.5 keV, a scanningregion was 200×200 μm, a detected region was 104 μmφ, secondary ionpolarity was positive). As a result, the surface concentration was1.0×10²⁰ atms/cm³, the maximum concentration was 1.4×10²⁰ atms/cm³, thedepth at which the maximum concentration was obtained was 0.07 μm, andthe diffusion depth was 0.5 μm. The sheet resistance of the emitterlayer was about 50 Ω/□.

Example 1

Example 1 is the case of extending the thermal oxidation time to 90minutes in Comparative Example 1.

The substrate formed with the emitter layer as described above was setin an oxygen atmosphere and thermally oxidized at a temperature of 1000°C. for 90 minutes, to form a silicon oxide layer on each surface of thesubstrate with a thickness of 80 nm. By the thermal treatment at thistime, boron diffused in the emitter layer was re-diffused. The diffusionprofile of boron in the emitter layer after the re-diffusion wasmeasured by SIMS, resulting in that the surface concentration was5.0×10¹⁹ atms/cm³, the maximum concentration was 7×10¹⁹ atms/cm³, thedepth at which the maximum concentration was obtained was 0.1 μm, andthe diffusion depth was 1.0 μm. The sheet resistance of the emitterlayer was about 60 Ω/□.

Example 2

Example 2 is the case of extending the thermal oxidation time to 120minutes in Comparative Example 1.

The substrate formed with the emitter layer as described above was setin an oxygen atmosphere and thermally oxidized at a temperature of 1000°C. for 120 minutes, to form a silicon oxide layer on each surface of thesubstrate with a thickness of 100 nm. By the thermal treatment at thistime, boron diffused in the emitter layer was re-diffused. The diffusionprofile of boron in the emitter layer after the re-diffusion wasmeasured by SIMS, resulting in that the surface concentration was1.0×10¹⁹ atms/cm³, the maximum concentration was 1.2×10¹⁹ atms/cm³, thedepth at which the maximum concentration was obtained was 0.4 μm, andthe diffusion depth was 2.0 μm. The sheet resistance of the emitterlayer was about 70 Ω/□.

Example 3

Example 3 is the case of extending the thermal oxidation time to 180minutes in Comparative Example 1.

The substrate formed with the emitter layer as described above was setin an oxygen atmosphere and thermally oxidized at a temperature of 1000°C. for 180 minutes, to form a silicon oxide layer on each surface of thesubstrate with a thickness of 130 nm. By the thermal treatment at thistime, boron diffused in the emitter layer was re-diffused. The diffusionprofile of boron in the emitter layer after the re-diffusion wasmeasured by SIMS, resulting in that the surface concentration was5.0×10¹⁸ atms/cm³, the maximum concentration was 8.0×10¹⁸ atms/cm³, thedepth at which the maximum concentration was obtained was 0.7 μm, andthe diffusion depth was 2.3 μm. The sheet resistance of the emitterlayer was about 90 Ω/□.

Example 4

Example 4 is the case of extending the thermal oxidation time to 240minutes in Comparative Example 1.

The substrate formed with the emitter layer as described above was setin an oxygen atmosphere and thermally oxidized at a temperature of 1000°C. for 240 minutes, to form a silicon oxide layer on each surface of thesubstrate with a thickness of 150 nm. By the thermal treatment at thistime, boron diffused in the emitter layer was re-diffused. The diffusionprofile of boron in the emitter layer after the re-diffusion wasmeasured by SIMS, resulting in that the surface concentration was5.0×10¹⁷ atms/cm³, the maximum concentration was 7.0×10¹⁷ atms/cm³, thedepth at which the maximum concentration was obtained was 1.0 μm, andthe diffusion depth was 2.9 μm. The sheet resistance of the emitterlayer was about 280 Ω/□.

Comparative Example 2

Comparative Example 2 is the case of extending the thermal oxidationtime to 300 minutes in Comparative Example 1.

The substrate formed with the emitter layer as described above was setin an oxygen atmosphere and thermally oxidized at a temperature of 1000°C. for 300 minutes, to form a silicon oxide layer on each surface of thesubstrate with a thickness of 160 nm. By the thermal treatment at thistime, boron diffused in the emitter layer was re-diffused. The diffusionprofile of boron in the emitter layer after the re-diffusion wasmeasured by SIMS, resulting in that the surface concentration was3.0×10¹⁷ atms/cm³, the maximum concentration was 5.0×10¹⁷ atms/cm³, thedepth at which the maximum concentration was obtained was 1.1 μm, andthe diffusion depth was 3.3 μm. The sheet resistance of the emitterlayer was about 320 Ω/□.

<Second Common Step>

The resist paste was screen-printed on the emitter layer formed place ofthe silicon oxide layer formed in the steps of Comparative Example 1 or2, or Examples 1, 2, 3, or 4, and heated at a temperature of 100° C. tobe dried. In this context, as the resist paste, Paste 185 manufacturedby Lektrachem Ltd. was used. The substrate printed with the resist pastewas soaked into a 2% hydrofluoric acid aqueous solution to remove thesilicon oxide layer other than the emitter layer formed place (a placefor forming the BSF layer), and thereafter soaked into acetone to removethe resist paste.

Subsequently, on the back surface of each of the substrate, from whichthe silicon oxide layer had been partially removed, the thermaldiffusion treatment was performed in a POCl₃-gas atmosphere at atemperature of 930° C. for 20 minutes, to diffuse phosphorus in theplace where the silicon oxide layer had been removed and form a glasslayer and an n-type diffusion layer as the BSF layer. The formed n-typediffusion layer had a sheet resistance of about 30Ω/□ and a diffusiondepth of 0.5 μm. Thereafter, these substrates were soaked into a 25%hydrofluoric acid aqueous solution, and then cleaned with deionizedwater and dried to remove the silicon oxide layer and the glass layer.

Then, the resist paste was screen-printed on the entire back surface ofthe substrate, and heated at a temperature of 100° C. to be dried. Inthis context, as the resist paste, Paste 185 manufactured by LektrachemLtd. was used. The substrate was chemically etched by use of a solutioncontaining 2% of potassium hydroxide and 2% of IPA at 70° C. for fiveminutes, and then cleaned with deionized water and dried to form atexture structure on the light receiving surface of the substrate.Thereafter, the substrate was soaked into acetone to remove the resistpaste.

Subsequently, by the plasma CVD method using SiH₄, NH₃, and N₂, siliconnitride layers as the antireflection films with passivation propertieswere formed having a thickness of 100 nm on both surfaces of thesubstrate.

<Third Common Step>

The conductive silver paste was printed on the emitter layer of thesubstrate, subjected to the above treatment so far, by the screenprinting method and then dried at 150° C. As the conductive silverpaste, SOL9383M manufactured by Heraeus Holding was used. Further, theconductive silver paste was printed on the BSF layer of the substrate bythe screen printing method using a plate that has 325 mesh, an emulsionthickness of 20 μm, and a linear opening with a width of 50 μm, and thepaste was then dried at 150° C. The printed conductive silver paste wasfired at the maximum temperature of 800° C. for five seconds to form anelectrode, and produce the back contact type solar cell according toeach of the comparative examples and examples.

<Implementation Result 1>

Table 1 shows an average conversion efficiency, an average short circuitcurrent density, an average open voltage, and an average fill factor ofeach 100 back contact type solar cells produced through steps ofComparative Example 1 or 2, or Examples 1, 2, 3, or 4.

TABLE 1 Impurity Average Average Average Average surface Diffusion Sheetconversion short circuit open fill concentration depth resistanceefficiency current density voltage factor (atms/cm³) (μm) (Ω/□) (%)(mA/cm²) (V) (%) Comparative 1 × 10²⁰ 0.5 50 19.0 38.1 0.641 77.9example 1 Example 1 5 × 10¹⁹ 1.0 60 19.5 38.5 0.646 78.3 Example 2 1 ×10¹⁹ 2.0 70 19.5 38.5 0.648 78.1 Example 3 5 × 10¹⁸ 2.3 90 19.4 38.40.651 77.7 Example 4 5 × 10¹⁷ 2.9 280 19.2 38.5 0.651 76.8 Comparative 3× 10¹⁷ 3.3 320 19.0 38.3 0.649 76.4 example 2

Setting the surface concentration of the impurities in the emitter layerto not higher than 5×10¹⁹ atms/cm³ (Examples 1 to 4) enabled enhancementof the conversion efficiency as compared with the case of theconventional product having a high surface concentration (ComparativeExample 1). This is considered to be because the reverse saturationcurrent density decreases and the open voltage increases due to thedecrease in surface concentration of the impurities. A short-circuitcurrent also increases at this time, and this is considered to beinfluenced by an increase in lifetime of the substrate with progress ofgettering of the metal impurities into the diffusion layer due toextension of the oxidation time. However, when the surface concentrationis made excessively low as in Comparative Example 2, lateral flowresistance of the emitter layer becomes large to prevent sufficientenhancement of the conversion efficiency. For this reason, the upperlimit of the surface concentration is preferably set to 5×10¹⁹ atms/cm³which is lower than the surface concentration of the conventionalproduct shown in Comparative Example 1, and the lower limit thereof ispreferably set to 5×10¹⁷ atms/cm³. Further, from the results of Examples1 to 4, the lower limit of the diffusion depth is preferably set to notsmaller than 1 μm which is larger than the diffusion depth of theconventional product shown in Comparative Example 1, and the upper limitthereof is preferably set to not larger than 2.9 μm. Moreover, from theresults of Examples 1 to 4, the lower limit of the sheet resistance ispreferably set to 60Ω/□, and the upper limit thereof is preferably setto 280Ω/□. However, when the upper limit is set to larger than 150Ω/□,variation in sheet resistance becomes very large at the time ofmass-production to make the control difficult, and hence the upper limitis more preferably set to 150 Ω/□.

Next, an influence exerted on the conversion efficiency by a change inshape of the electrode for making the contact resistance small isverified.

Comparative Example 3

The conductive silver paste was printed by the screen printing method onthe BSF layer of the substrate subjected to the processing in Example 2and the second common step, and the paste was then dried at 150° C. Asthe conductive silver paste, SOL9412 manufactured by Heraeus Holding wasused. Main solid components of SOL9412 are silver and glass frit, and analuminum powder is not added thereto.

Further, the conductive silver paste was printed on the emitter layer ofthe substrate by the screen printing method using a plate that has 360mesh, an emulsion thickness of 10 μm, and a linear opening with a widthof 60 μm, and the paste was then dried at 150° C. Thereafter, thesubstrate printed with the conductive silver paste was fired at themaximum temperature of 800° C. for five seconds to form an electrode.The electrode after formed had a width of about 70 μm, a thickness ofabout 8 μm, and a cross-sectional area of about 250 μm². The electrodeon the semiconductor substrate as thus formed was removed byhydrofluoric acid and nitric acid, and the penetration depth into thediffusion layer was measured by SEM, to recognize no penetrating place.

Comparative Example 4

Comparative Example 4 is the case of applying, as the conductive silverpaste, one added with the aluminum powder in Comparative Example 3.

The conductive silver paste was printed by the screen printing method onthe BSF layer of the substrate subjected to the processing in Example 2and the second common step, and the paste was then dried at 150° C. Asthe conductive silver paste, SOL9383M manufactured by Heraeus Holdingwas used. Main solid components of SOL9383M are silver, the glass frit,and the aluminum powder.

Further, the conductive silver paste was printed on the emitter layer ofthe substrate by the screen printing method using a plate that has 360mesh, an emulsion thickness of 10 μm, and a linear opening with a widthof 60 μm, and the paste was then dried at 150° C. Thereafter, thesubstrate printed with the conductive silver paste was fired at themaximum temperature of 800° C. for five seconds to form an electrode.The electrode after formed had a width of about 70 μm, a thickness ofabout 8 μm, and a cross-sectional area of about 250 μm². The electrodeon the semiconductor substrate as thus formed was removed byhydrofluoric acid and nitric acid, and the penetration depth into theemitter layer was measured by SEM, to find that the maximum value of thepenetration depth was 0.05 μm.

Example 5

Example 5 is the case of setting the mesh and the emulsion thickness, tobe applied to the screen printing in Comparative Example 4, to 325 meshand 20 μm, respectively.

The conductive silver paste was printed by the screen printing method onthe BSF layer of the substrate subjected to the processing in Example 2and the second common step, and the paste was then dried at 150° C. Asthe conductive silver paste, SOL9383M manufactured by Heraeus Holdingwas used.

Further, the conductive silver paste was printed on the emitter layer ofthe substrate by the screen printing method using a plate that has 325mesh, an emulsion thickness of 20 μm, and a linear opening with a widthof 60 μm, and the paste was then dried at 150° C. Thereafter, thesubstrate printed with the conductive silver paste was fired at themaximum temperature of 800° C. for five seconds to form an electrode.The electrode after formed had a width of about 70 μm, a thickness ofabout 12 μm, and a cross-sectional area of about 350 pmt. The electrodeon the semiconductor substrate as thus formed was removed byhydrofluoric acid and nitric acid, and the penetration depth into thediffusion layer was measured by SEM, to find that the maximum value ofthe penetration depth was 0.1 μm.

Example 6

Example 6 is the case of setting the mesh and the emulsion thickness, tobe applied to screen printing in Comparative Example 4, to 290 mesh and30 μm, respectively.

The conductive silver paste was printed by the screen printing method onthe BSF layer of the substrate subjected to the processing in Example 2and the second common step, and the paste was then dried at 150° C. Asthe conductive silver paste, SOL9383M manufactured by Heraeus Holdingwas used.

Further, the conductive silver paste was printed on the emitter layer ofthe substrate by the screen printing method using a plate that has 290mesh, an emulsion thickness of 30 μm, and a linear opening with a widthof 60 μm, and the paste was then dried at 150° C. Thereafter, thesubstrate printed with the conductive silver paste was fired at themaximum temperature of 800° C. for five seconds to form an electrode.The electrode after formed had a width of about 70 μm, a thickness ofabout 15 μm, and a cross-sectional area of about 600 pmt. The electrodeon the semiconductor substrate as thus formed was removed byhydrofluoric acid and nitric acid, and the penetration depth into thediffusion layer was measured by SEM, to find that the maximum value ofthe penetration depth was 0.9 μm.

Example 7

Example 7 is the case of setting the mesh and the emulsion thickness, tobe applied to the screen printing in Comparative Example 4, to 250 meshand 30 μm, respectively.

The conductive silver paste was printed by the screen printing method onthe BSF layer of the substrate subjected to the processing in Example 2and the second common step, and the paste was then dried at 150° C. Asthe conductive silver paste, SOL9383M manufactured by Heraeus Holdingwas used.

Further, the conductive silver paste was printed on the emitter layer ofthe substrate by the screen printing method using a plate that has 250mesh, an emulsion thickness of 30 μm, and a linear opening with a widthof 60 μm, and the paste was then dried at 150° C. Thereafter, thesubstrate printed with the conductive silver paste was fired at themaximum temperature of 800° C. for five seconds to form an electrode.The electrode after formed had a width of about 70 μm, a thickness ofabout 15 μm, and a cross-sectional area of about 950 pmt. The electrodeon the semiconductor substrate as thus formed was removed byhydrofluoric acid and nitric acid, and the penetration depth into thediffusion layer was measured by SEM, to find that the maximum value ofthe penetration depth was 1.5 μm.

Example 8

Example 8 is the case of setting the mesh and the emulsion thickness, tobe applied to the screen printing in Comparative Example 4, to 250 meshand 40 μm, respectively.

The conductive silver paste was printed by the screen printing method onthe BSF layer of the substrate subjected to the processing in Example 2and the second common step, and the paste was then dried at 150° C. Asthe conductive silver paste, SOL9383M manufactured by Heraeus Holdingwas used.

Further, the conductive silver paste was printed on the emitter layer ofthe substrate by the screen printing method using a plate that has 250mesh, an emulsion thickness of 40 μm, and a linear opening with a widthof 60 μm, and the paste was then dried at 150° C. Thereafter, thesubstrate printed with the conductive silver paste was fired at themaximum temperature of 800° C. for five seconds to form an electrode.The electrode after formed had a width of about 70 μm, a thickness ofabout 15 μm, and a cross-sectional area of about 1050 pmt. The electrodeon the semiconductor substrate as thus formed was removed byhydrofluoric acid and nitric acid, and the penetration depth into thediffusion layer was measured by SEM, to find that the maximum value ofthe penetration depth was 1.9 μm.

<Implementation Result 2>

Using the substrate subjected to the processing shown in ComparativeExample 3 or 4, or Examples 5, 6, 7, or 8, each 100 back contact typesolar cells according to each of the comparative examples and exampleswere produced, to measure an average conversion efficiency, an averageshort circuit current density, an average open voltage, and an averagefill factor. Table 2 shows results of the measurement.

TABLE 2 A: Surface concentration Average B: Maximum concentrationContain Cross Average short circuit Average Average C: Maximumconcentration depth conductive section of Penetration conversion currentopen fill D: Diffusion depth paste electrode depth efficiency densityvoltage factor E: Sheet resistance aluminum (μm²) (μm) (%) (mA/cm²) (V)(%) Comparative A = 1 × 10¹⁹ atms/cm³ No 250 0 13.3 37.5 0.638 55.7example 3 B = 1.2 × 10¹⁹ atms/cm³ Comparative C = 0.4 μm Yes 250 0.0518.9 38.1 0.640 77.7 example 4 D = 2.0 μm Example 5 E = 70 Ω/□ Yes 3500.1 19.3 38.1 0.642 78.9 Example 6 Yes 600 0.9 19.4 38.1 0.641 79.4Example 7 Yes 950 1.5 19.5 38.1 0.642 79.7 Example 8 Yes 1050 1.9 19.438.0 0.641 79.7

It is found from comparison between Comparative Example 3 andComparative Example 4 that in order to penetrate the electrode into theemitter layer, it is preferable to add the aluminum powder to theconductive paste in addition to silver and the glass frit, to form asintered body of these. Further, it is found that even a littlepenetration of the electrode into the emitter layer greatly improves theconversion efficiency.

From comparison between Comparative Example 4 and Examples 5 to 8, it isfound that the conversion efficiencies in the examples are better.Especially, the conversion efficiencies in Examples 6 to 8 areexcellent. This is because the electrode with a wide cross-sectionalarea penetrates deeply, exceeding the maximum concentration depth of theimpurities, thereby making the contact resistance stable low andsignificant deterioration in fill factor hardly occur.

Even when the cross-sectional area of the electrode is set to notsmaller than 1000 μm² as in Example 8, it does not lead to a significantincrease in conversion efficiency as compared with the case when thecross-sectional area of the electrode was not larger than 1000 μm².Rather, it can be said that the upper limit of the cross-sectional areaof the electrode is preferably set to approximately 1000 μm² consideringa cost increase due to an increase in amount of the electrode used, therisk of the electrode penetrating through the emitter layer due toexcessively large penetration depth to cause a decrease in parallelresistance, and further considering the tendency of the formed silverelectrode to be peeled from the substrate due to excessive progress ofsintering of the silver powder caused by the increase in cross-sectionalarea of the electrode.

For this reason, from the results of Examples 5 to 8, it is preferableto set the penetration depth to not smaller than 0.1 μm and not largerthan 1.9 μm, and set the cross-sectional area to not smaller than 350μm² and not larger than 1000 μm². The cross-sectional area of theelectrode and the penetration depth can be appropriately adjusted bychanging the mesh roughness and the emulsion thickness.

The above implementation result for Comparative Example 3 or 4, orExample 5, 6, 7, or 8 is a result concerning the case of forming theelectrode on the substrate formed with the emitter layer by the methodof Example 2, but a preferable penetration depth and a preferablecross-sectional area are similar even in the case of forming theelectrode on the substrate formed with the emitter layer by the methodof Example 1, 3, or 4. For this reason, in the light of the results ofExamples 1 to 4, the maximum value of the concentration of theimpurities in the emitter layer is preferably not lower than 7×10¹⁷atms/cm³ and not higher than 7×10¹⁹ atms/cm³, and the maximum value ofthe depth is preferably set to not smaller than 0.1 μm and not largerthan 1 μm from the top of the back surface of the substrate.

With such a configuration as above, it is possible to provide a highefficiency back contact type solar cell having favorable conversionefficiency and being manufacturable by a simple method with good yieldat low cost.

The high efficiency back contact type solar cell 200 produced inaccordance with the above embodiment can be used for a solar cellmodule. FIG. 7 is a schematic view that illustrates a configurationexample of a solar cell module 300. The solar cell module 300 has astructure where a plurality of high efficiency back contact type solarcells 200 are spread in the form of tiles. As for the plurality of highefficiency back contact type solar cells 200, each several to severaltens of mutually adjacent cells are electrically connected in series toconstitute a serial circuit called a string. FIG. 8 illustrates anoverview of the string. FIG. 8 corresponds to a schematic view of theinternal back surface side of the solar cell module 300 which isnormally unseen. In FIG. 8, fingers and bus bars are omitted for thesake of clarifying the description. For constituting the serial circuit,a P bus bar and an N bus bar of the mutually adjacent high efficiencyback contact type solar cells 200 are connected by lead wires 320. FIG.9 illustrates a sectional schematic view of the solar cell module 300.As described above, the string is configured by connecting the pluralityof high efficiency back contact type solar cells 200 by connection ofthe lead wire 320 to the bus bars 310. The string is sealed typically bya translucent filler 330 such as EVA (ethylene vinyl acetate), thenon-light receiving surface (back surface) side is covered by aweatherproof resin film 340 such as PET (polyethylene terephthalate),and the light receiving surface is covered by a light receiving surfaceprotecting material 350 having translucency and high mechanicalstrength, such as soda-lime glass. As the filler 330, polyolefin,silicone, or the like can be used other than EVA.

A plurality of solar cell modules can be connected to constitute thephotovoltaic power generation system. FIG. 10 is a schematic view thatillustrates a configuration example of a photovoltaic power generationsystem 400 formed by coupling a plurality of solar cell modules 300 madeup of a plurality of high efficiency back contact type solar cells 200of the present invention. The photovoltaic power generation system 400is formed by coupling a plurality of solar cell modules 300 in series bywiring 410 and supplies generated power to an external load circuit 430through an inverter 420. Although not illustrated in FIG. 10, thephotovoltaic power generation system 400 may further include a secondarybattery for storing the generated power.

Note that the present invention is not restricted to the aboveembodiments and examples, and one which has substantially the sameconfiguration as the technical idea recited in the claims and exerts asimilar function effect thereto is included in the technical scope ofthe present invention even if any change has been made.

REFERENCE SIGNS LIST

-   100 Back contact type solar cell-   101 Semiconductor substrate-   102 Protective layer-   103 Resist paste-   104 Emitter layer-   105 Glass layer-   106 BSF layer-   107, 108 Antireflection films with passivation properties-   109, 110 Electrode-   200 High efficiency back contact type solar cell-   300 Solar cell module-   310 Bus bar-   320 Lead wire-   330 Filler-   340 Weatherproof resin film-   350 Light receiving surface protecting material-   400 Photovoltaic power generation system-   410 Wiring-   420 Inverter-   430 External load circuit

1. A high efficiency back contact type solar cell in which an impuritydiffusion layer where second conductive type impurities are diffused isformed on a back surface, as a non-light receiving surface, of a firstconductive type semiconductor substrate, and an electrode in contactwith the impurity diffusion layer is provided, wherein a surfaceconcentration of the impurities in the impurity diffusion layer is notsmaller than 5×10¹⁷ atms/cm³ and not larger than 5×10¹⁹ atms/cm³, and adiffusion depth of the impurities in the impurity diffusion layer is notsmaller than 1 μm and not larger than 2.9 μm from a top of the backsurface.
 2. The high efficiency back contact type solar cell accordingto claim 1, wherein sheet resistance of the impurity diffusion layer isnot smaller than 60Ω/□ and not larger than 150 Ω/□.
 3. The highefficiency back contact type solar cell according to claim 1, wherein amaximum value of an impurity concentration of the impurity diffusionlayer is not lower than 7×10¹⁷ atms/cm³ and not higher than 7×10¹⁹atms/cm³, the impurity concentration of the impurity diffusion layerbecomes the maximum value at a position not less than 0.1 μm and notmore than 1 μm deep from the top of the back surface, the electrode is asintered body containing at least glass flit, silver, and aluminum, across-sectional area of the electrode is not smaller than 350 μm² andnot larger than 1000 μm², and the electrode partially penetrates intothe impurity diffusion layer, and a depth of the penetration is notsmaller than 0.1 μm and not larger than 1.9 μm from the top of the backsurface.
 4. The high efficiency back contact type solar cell accordingto claim 1, wherein the first conductive type is an n-type, and thesecond conductive type is a p-type.
 5. A solar cell module, formed byelectrically connecting a plurality of high efficiency back contact typesolar cells according to claim
 1. 6. A photovoltaic power generationsystem, formed by electrically connecting a plurality of solar cellmodules according to claim 5.